My older sister Liz always hated New Year. I remember her sitting self-consciously in a pub corner while our chums and I staggered in time to "Auld Lang Syne". Four years, a husband and two children later, she sat in another pub corner, but she had a companion. The girl she talked to was a Jehovah's Witness who explained that she didn't celebrate New Year, Christmas, or birthdays because of her religion, and it made my sister curious.

In April 1987, Liz went to her first Jehovah's Witness meeting and I felt slightly apprehensive. Witnesses were up there with Moonies as a nutty religious cult. Wouldn't they rather die than have a blood transfusion? Don't they get brainwashed and ostracise themselves from society? Would my nephews ever have a normal childhood?

Liz talked about Witnesses constantly, mainly because she wanted to share her new-found enthusiasm. I tried my best to ignore it, but in March 1990 my level-headed, rational sister was baptised a Jehovah's Witness. I was terrified she would become ill and refuse a blood transfusion. My mother, a devout Christian, was completely panic-stricken and would call me with predictions of disaster. Fuelled by her friends in church, she was convinced that Liz and her children would abandon us and that she'd committed herself and her children to eternal damnation.

In retrospect, the whole idea was preposterous, but at the time it was deeply unsettling. Even though I didn't really believe in a God, what if he did exist and what if my mother was right? Then Liz would call and try to assure me that she wasn't intent on spending an eternity in a furnace or pulling away from her family.

It felt like a battle for my soul. Every week they would both phone me, either to reveal some fresh horror awaiting Witnesses in the after-life, or to exasperate me the way only the freshly converted can. I considered going to live in a primeval Welsh forest and becoming a pagan. At least you get to be a priestess and wear full-length gowns.

Visits to my sister were miserable occasions. Every conversation was twisted around to Joho-speak. Gone were the lively conversations on politics, culture, friends and family, and copies of Watchtower were thrust in my hands. My nephews were trotted off to bible meetings looking like computer salesmen (tie, three-piece suit, neatly combed hair). I felt I had lost my sister and my (by now) four precious nephews to some fabricated, corporate American religion.

My sister and my mother were just about on speaking terms, mainly because Liz made every effort to keep in touch. My mother was still deeply distressed, and they would always row about some minor biblical detail. I finally had enough. I asked them to both sit down and rationally discuss their religious differences. What started as a calm conversation tumbled into farce. My mother called Liz a heathen for not believing in the Holy Spirit or that Jesus was the son of God and accused her of rank stupidity. Liz in turn accused Mum of intolerance, over-reaction and an inability to listen. It was like a mini religious war in my bedroom and I could see no solution.

The only glimmer of hope was my brother-in-law. Tony is an agnostic and remained unaffected by his wife's religion. He did notice, however, that it made her happier (something my mother and I had failed to spot). Between them they decided that the children should only go to meetings if they wanted to and that if they needed a blood transfusion, he would authorise it, which was a huge relief to all. It was Tony who first stemmed the tide of panic. "This isn't Invasion of the Body Snatchers, you know," he'd say. "If anybody can tell if my wife has fundamentally changed, it's me and I know that she hasn't."

Once Liz's religious zeal had died down a little, I realised he was right. She hadn't suffered a sense of humour failure, lost interest in politics or even beauty products. Liz does two hours of ministry a week - knocking on doors to spread the word - and for a few months she became an Avon Lady. People didn't know whether she was selling religion or a new range of lipstick. I soon realised that I could ask her questions about her religion without being subjected to half-hour lectures and order great lipsticks to boot.

I learnt that Jehovah's Witnesses believe that your body, soul and mind are a complete spiritual unit and that when you die you just stop existing. They believe that on Judgement Day, God will bring back to life all the "good people" and put them back on earth. They believe that 144,000 Witnesses will go to a sort of heavenly parliament, while the rest will live on an earthly paradise. Unbelievers will just cease to exist. Of course, it sounds completely barmy, but to the atheist, Christianity sounds like the concoction of a mad man. It is a lot to get your head around when your idea of heaven is a vodka Martini and nice warm body, but I'm trying to understand.

I have noticed that Witnesses have fun. They go camping and get married on a beach and have big parties. They listen to Nirvana and U2, drink and take pride in their appearances. Witness children are encouraged to study the bible, and from the earliest age my nephews gave mini-lectures to and answered questions from their congregation, and they excelled at school (when my sister and I went to church we used to fight over wine gums and laugh at old ladies' hats). Even my education-obsessed mother overcame her fears and saw my 10-year-old nephew address his congregation on a section of the bible. She couldn't help but be impressed.

By becoming a Jehovah's Witness, my sister was introduced to a network of friends who would support and help her everyday life. If the car broke down, she could trust a Witness to fix it without ripping her off. If she needed a baby-sitter at short notice, there was always someone available, and my nephews formed close bonds with other Witness children. These people - of whom I initially disapproved because of their crisply co-ordinated clothes, white toothy smiles and goody-goody behaviour - are actually okay. A little condescending, perhaps, but certainly not brainwashed fools.

My two oldest nephews have grown up into good-looking surfing dudes who think it's neither funny or clever to take drugs, smoke or have sex before marriage (on occasion, I find it hard to believe they are related to me). They have seen Trainspotting, they go for a drink down the pub, have pictures of babes on their wall and can catch a wave with the best of them. The two younger boys are just normal, mischievous, talented children.

They have summer parties to make up for lost birthdays, they get money at Christmas and presents when they least expect them. My sister gives me little gifts because she wants to, not because I'm a year older. And Tony sticks an old, ragged tinsel tree in the corner, sends Christmas cards to relatives and both their mothers come over for the holidays. Edward, my sister's 17-year old son, is now a pioneer (someone who does 25 hours a week of ministry work), but only after his parents made sure he wasn't doing it because he felt he had to. This is a boy who got 10 top-notch GCSEs, but it's his decision and I respect it.

My mother has finally accepted, with encouragement from her son-in-law and myself, that Liz will not run off to the Witness equivalent of Salt Lake City, or be tempted to follow the Hale Bopp comet. She's just found a religion that makes sense of her life and a moral code for her children in a world she sees as increasingly selfish and materialistic. She believes that she will be safe in the after-life and in these unspiritual times, maybe she's found something very precious indeed. I, however, will keep drinking the vodka Martinis.